Question

应用程序在此活动的启动时崩溃，但如果在单击按钮时通过onClick（）方法调用设置适配器的代码，则代码运行正常... 如果代码在onCreateview（）中使用而不是onCreate（），则静态引用不能用于静态方法findViewById（）

public class DeviceList extends ActionBarActivity {

BluetoothAdapter mBluetoothAdapter = BluetoothAdapter.getDefaultAdapter();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_device_list);

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }

    Set<BluetoothDevice> pairedDevices = mBluetoothAdapter.getBondedDevices();
    ListView listview = (ListView) findViewById(R.id.listdevices);
    ArrayList<String> list = new ArrayList<String>();
    if(pairedDevices.size()>0) {
        for(BluetoothDevice device: pairedDevices) {
            list.add(device.getName() + "\n" + device.getAddress());
        }
    }
    ArrayAdapter adapter = new ArrayAdapter(this,android.R.layout.simple_list_item_1,    list);

    listview.setAdapter(adapter);
}

xml文件是

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.checkingbluetooth.DeviceList$PlaceholderFragment"
android:background="#C0C0C0" 
android:orientation="vertical">



<ListView
    android:id="@+id/listdevices"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    />



</LinearLayout>

日志

07-09 15:08:07.414: W/dalvikvm(22973): threadid=1: thread exiting with uncaught exception (group=0x41e6cae0)
07-09 15:08:07.414: E/AndroidRuntime(22973): FATAL EXCEPTION: main
07-09 15:08:07.414: E/AndroidRuntime(22973): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.checkingbluetooth/com.example.checkingbluetooth.DeviceList}: java.lang.NullPointerException
07-09 15:08:07.414: E/AndroidRuntime(22973):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2187)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2237)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at android.app.ActivityThread.access$600(ActivityThread.java:146)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1239)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at android.os.Handler.dispatchMessage(Handler.java:99)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at android.os.Looper.loop(Looper.java:137)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at android.app.ActivityThread.main(ActivityThread.java:5085)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at java.lang.reflect.Method.invokeNative(Native Method)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at java.lang.reflect.Method.invoke(Method.java:511)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at dalvik.system.NativeStart.main(Native Method)
07-09 15:08:07.414: E/AndroidRuntime(22973): Caused by: java.lang.NullPointerException
07-09 15:08:07.414: E/AndroidRuntime(22973):    at com.example.checkingbluetooth.DeviceList.onCreate(DeviceList.java:48)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at android.app.Activity.performCreate(Activity.java:5104)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
07-09 15:08:07.414: E/AndroidRuntime(22973):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2151)
07-09 15:08:07.414: E/AndroidRuntime(22973):    ... 11 more

Answer 1

我认为您的listview将为null，因为您的列表视图应位于PlaceholderFragment类内。请在placeholderfragment的布局中添加列表视图中做一件事，然后尝试它会起作用，谢谢; - ）

Answer 2

你的位置：

R.id.container

如果您的代码超过了第一个条件，它就会崩溃。如果你不使用它，请将其删除。

Answer 3

您将获得NullPointerException

引起：java.lang.NullPointerException at com.example.checkingbluetooth.DeviceList.onCreate（DeviceList.java:48）

我在DeviceList.java:48的猜测是

ListView listview = (ListView) findViewById(R.id.listdevices);

原因listdevices ListView位于您的片段布局中，这就是为什么 null 将ListView放在activity_device_list中。我认为这将解决您的问题

listview.setadapter（adapter）在oncreate（）中导致空指针异常

3 个答案: